Metallacycles Model for Initiation and Propagation

Experimental results supporting the metallacycles model for initiation and propagation have also been proposed [77,99,135,136]. As already discussed, Ghiotti et al. [53] and Zecchina et al. [77] did not obtain IR spectral evidence indicating the presence of vinyl or methyl groups in the firsts stages... 

From the results discussed so far, it is evident that only CH2 groups have been observed in the very early stages of the ethylene polymerization reaction. Of course, this could be due to formation of metallacycles, but can be also a consequence of the high TOP which makes the observation of the first species troublesome. To better focalize the problem it is useful to present a concise review of the models proposed in the literature for ethylene coordination, initiation, and propagation reactions. 

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